udp6: set rx_dst_cookie on rx_dst updates

Currently, in the udp6 code, the dst cookie is not initialized/updated
concurrently with the RX dst used by early demux.

As a result, the dst_check() in the early_demux path always fails,
the rx dst cache is always invalidated, and we can't really
leverage significant gain from the demux lookup.

Fix it adding udp6 specific variant of sk_rx_dst_set() and use it
to set the dst cookie when the dst entry is really changed.

The issue is there since the introduction of early demux for ipv6.

CRs-Fixed: 2122802
Change-Id: I607ee86f4b53dd422dda81d88fdfb03a73647676
Fixes: 5425077d73e0 ("net: ipv6: Add early demux handler for UDP unicast")
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Git-commit: 64f0f5d18a47c703c85576375cc010e83dac6a48
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>
diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
index 7e5d8ed..2db6808 100644
--- a/net/ipv6/udp.c
+++ b/net/ipv6/udp.c
@@ -754,6 +754,15 @@
 	return 0;
 }
 
+static void udp6_sk_rx_dst_set(struct sock *sk, struct dst_entry *dst)
+{
+	if (udp_sk_rx_dst_set(sk, dst)) {
+		const struct rt6_info *rt = (const struct rt6_info *)dst;
+
+		inet6_sk(sk)->rx_dst_cookie = rt6_get_cookie(rt);
+	}
+}
+
 int __udp6_lib_rcv(struct sk_buff *skb, struct udp_table *udptable,
 		   int proto)
 {
@@ -803,7 +812,7 @@
 		int ret;
 
 		if (unlikely(sk->sk_rx_dst != dst))
-			udp_sk_rx_dst_set(sk, dst);
+			udp6_sk_rx_dst_set(sk, dst);
 
 		ret = udpv6_queue_rcv_skb(sk, skb);
 		sock_put(sk);